This invention deals with the machining field and especially the adaptations ensuring the complete machining of a crankshaft in the best possible conditions.
Crankshafts are mechanical pieces having the shape of a shaft, which ensure the transmission of the alternate linear motion of the piston and connecting rod of a thermal engine to rotating motion. The pieces are classically manufactured by forging or molding before being subjected to a precise machining.
In the case of a thermal four-cylinder engine, the crankshaft is provided with four bearings, by means of which it is connected to the casing through a pivot link, four crank pins placed parallel to the rotation axis of the crankshaft and intended to drive the four connecting rods and four counterweights ensuring a balance of the masses of the crankshaft with respect to its rotation axis with the four crank pins and the eight arms that offset them from the rotation axis of the crankshaft.
Classically, the beginning of the manufacturing chain by matter removal of a crankshaft includes the following steps:
definition of the points of reference,
machining of the ends,
machining of the bearings,
machining of the crank pins, and
manufacturing of the oil holes, prior to various finishing operations as well as the final balancing through dynamic measurement and eventual equilibration through matter removal.
Improvements in the determination of the points of reference through centering of the crankshaft as described in the French patent 2 761 129 led the applicant to study and implement a succession of different operations in the classical machining method of a crankshaft.
Indeed, machining operations being expensive, it is essential to limit their numbers and their duration, while respecting the geometric and dynamic constraints defined by the car manufacturers.
These studies led to the concept of a new machining method allowing the optimization of the crankshaft manufacturing. According to the main feature of the invention, the machining method of the invention is remarkable in that it consists in performing the dynamic measurement and matter removal operations for balancing purposes before the finishing operations.
This feature is particularly judicious in that it ensures the balancing of the crankshaft before the operations where the unbalanced mass and the matter removal will not involve very much. Indeed, the finishing and superxe2x80x94finishing operations, as indicated by their names, do not remove a lot of matter and therefore do not impact in a significant manner on the unbalanced mass of the crankshaft. In all cases, matter removal on such operations can be parameterized.
This original setting of the balancing operation in the machining method of a crankshaft of the invention provides several advantages. On one hand, it allows the suppression of the equilibration process by drilling the crankshafts at the end of their achievement chain through matter removal and, on the other hand, it guarantees a dynamic measurement of the masses during an operation being relatively close to the finishing operation of the points of reference, which ensures an easier and quicker correction if said definition of the points of reference had defined surfaces of reference that would require too much matter removal in the following operations.
The operation and devices of the dynamic measurement of the masses are not new, but the performance of this operation at that moment of the achievement chain through matter removal of a crankshaft is particularly new and inventive. This change of concept in the manufacturing of a crankshaft was only possible through the recent progress made in the proper definition of the points of reference of the crankshaft to be machined.
Indeed, the concept of the methods of the prior art was to perform the measurement and the balancing of a crankshaft only when the latter had gone through all the phases of matter removal, whether machining, semi-finishing or finishing. Thus, the whole machining chain could ensure the achievement of a bad machining on a crankshaft, because of badly defined points of reference for instance, without being able to detect this faulty crankshaft before the end of the manufacturing chain, that is during the final balancing operation. Therefore, the method according to the invention remedies not only this drawback of late detection of a faulty crankshaft but it also allows for its correction.
The fundamental concepts of the invention having been described hereabove in their most elementary form, other details and features will come through more clearly by reading the following description, giving as a non restricting example an embodiment of a machining operation of a crankshaft with original setting of the balancing operation according to the invention.